Semi-permeable membranes are used for extracting a solvent, for example water, from a solution or mixture and rejecting unwanted solutes or solids, such as salts and other contaminants. The semi-permeable membranes may be cross-linked polyamide composite membranes that comprise a separation layer supported upon a porous substrate. The separation layers are thin to maximize membrane performance while the porous substrate provides the necessary structural support. The separation layer may be a thin-film polyamide layer that is formed, coated or deposited on a porous polysulfone support. The membrane may be used in reverse osmosis or nanofiltration processes.
The separation layer may be formed on the porous support by various techniques, including interfacial polymerization. Interfacial polymerization forms a thin-film polyamide at an interface between an aqueous solution and a water immiscible organic solvent. The interface is positioned at or near the surface of the porous substrate to facilitate depositing the thin-film on the substrate.
Isopropyl alcohol can be added to the aqueous solution during interfacial polymerization. The isopropyl alcohol improves permeability characteristics but at the expense of solute rejection characteristics. Additionally, significant amounts of isopropyl alcohol are required to achieve the permeability improvements, which may increase health and safety risks that are associated with the volatility of isopropyl alcohol.